There have in the past been provided a wide variety of counterbalance holding valves for controlling the flow of fluid relative to a hydraulic actuator. In many applications, such as mobile equipment, reciprocating piston and cylinder actuators are employed for raising and lowering loads under the influence of gravity. Conventionally, four-way valves are provided for selectively porting fluid to either side of the actuator so that the load may be moved in either direction.
In such applications, it has been found that there is a tendency for the load to overrun under the influence of gravity during load lowering, resulting in the too rapid movement of the load and cavitation in the high pressure side of the actuator.
One method of controlling load lowering is a pilot-operated counterbalance valve in the actuator return line that variably restricts flow from the actuator in an attempt to maintain uniform load lowering. These prior counterbalance valves are pilot actuated to an open position by a fluid operable pilot piston that receives fluid from the passage delivering fluid to the load lowering side of the actuator. Thus, as the pressure in the load lowering side of the actuator decreases, indicating a tendency to cavitate and load overrunning, pilot pressure will fall permitting the counterbalance valve to further restrict return flow from the actuator, thereby slowing down the movement of the load.
These prior counterbalance valves have been found to have several disadvantages. They have a tendency to chatter under load fluctuation, they respond slowly to changes in load speed and also require high pilot pressure to maintain the counterbalance valve off its seat since the valve is subjected to full load pressure.
It is the primary object of the present invention to eliminate or minimize the above-described problems in counterbalance valves.